When measuring the length of an elongated object such as a communication cable or power cable, usually the length is measured while the elongated object is in a moving state. The method of measurement is to measure the lengths of cable for standard length measurement distances and to add up the lengths measured for these standard length measurement distances. Cables are extremely long in length, so if the measurement errors for each standard length measurement distance are accumulated, they become a massive error in terms of the length of the overall cable. Therefore, the precision of measurement of the length of the cable in each standard length measurement distance must be made with extremely high precision.
Recently, attempts have been made to commercialize a prefabrication work process using cables measured with a high precision. In such a prefabrication work process, it is necessary to measure the cable length with an extremely high precision of, for example, 0.02 percent.
In the past, as a method for measuring cable length, there has been known the encoder system and the marking system.
The encoder system measures the length of a cable by bringing a single encoder wheel (rotational member) into contact with a moving cable, turning the encoder wheel by the movement of the cable, outputting pulses corresponding to the amount of movement of the cable from the encoder, and counting the pulses.
The encoder system has the advantages that it enables measurement of length with a simple construction and enables measurement of length even with short unmeasurable lengths. In the encoder system, however, the error is considerably large due to the slipping of the encoder wheel, the fluctuations in the outer diameter of the encoder wheel due to temperature changes, wear, etc., and the deformation of the surface of the cable. Thus, the precision of detection is at best 0.2 percent or so, and there is the problem that the high precision of 0.02 percent or so required for the prefabrication work process etc. cannot be obtained.
On the other hand, the marking system, for example, is disclosed in Japanese Unexamined Published Patent Application (Kokai) No. 57-28204. In this marking system, a marker and a sensor for detecting marks provided by the marker are provided and are separated by a standard length measurement distance in the direction of movement of the cable. When the sensor detects a mark, it outputs a detection signal to the length measurement apparatus body, which drives the marker to make a mark on the surface of the cable, to indicate the cable as having moved by exactly by the standard length measurement distance, and advances the counter by 1. The marks made by the marker are detected by the sensor. The operation of measuring the length of the cable for each standard length measurement distance is repeated and the standard length measurement distance S is multiplied with the final count N to calculate the overall length of the cable.
The marking system measures the length of a cable as a whole multiple of a predetermined standard length measurement distance, thus has the advantage of a higher precision of measurement than the encoder system. However, the marking system has the problems that it is impossible to measure lengths shorter than a standard length measurement distance, measurement error occurs when the distance between marks around when the positional marking precision of the marker on the moving cable is low, measurement error occurs when the speed of movement of the cable fluctuates, and measurement error occurs when the operational timing of the marker deviates due to the unevenness of the surface of the cable and the distance between marks fluctuates.
Further, whether by the encoder system or the marking system, the problem is encountered that it is impossible to check the factors behind the occurrence of such errors.